1. Field of the Invention
The present invention relates to a field emission device and more particularly, to a field emission device that is able to readily control the emission direction of electrons independent of the unbalance or asymmetry in pattern of a gate electrode.
2. Description of the Prior Art
Conventionally, various types of field emission devices have been developed; typical examples of which were reported by C. A. Spindt et al. in the article, Journal of Applied Physics, Vol. 47, No. 12, pp. 5248-5263, published in December 1976, and by H. F. Gray et al. in the article, 1986 IEDM Technical Digest, pp. 776-779, published in 1986.
An example of the conventional field emission devices is shown in FIG. 1, which includes a semiconductor substrate 31 having an upper main surface 31b and a lower main surface. or back surface 31a. The first and second main surfaces 31b and 31a are parallel to each other.
An insulating layer 32 is formed on the upper main surface 31b of the substrate 31. A conductive layer 42 is selectively formed on the insulating layer 32. The conductive layer 42 has a part serving as a gate electrode 33, a part serving as a bonding pad (not shown), and a part serving as an interconnection 38 for electrically interconnecting the gate electrode 33 and the bonding pad.
The gate electrode 33 has circular apertures or windows 33a arranged in a matrix array to expose the underlying insulating layer 32. The insulating layer 32 has circular penetrating holes 34 to expose the underlying upper main surface 31b of the substrate 31. The holes 34 are arranged at the locations just below the corresponding windows 33a of the gate electrode 33.
Cathodes 35, which are made of a conductive metal such as molybdenum (Mo), are formed on the exposed upper main surface 31b of the substrate 31 in the corresponding holes 34 of the insulating layer 32, respectively. Each of the cathodes 35 has a shape of a sharp-pointed cone. The tips of the cathodes 35 are located in the vicinity of the interface of the gate electrode 33 and the insulating layer 32.
A conductive layer 36, which is made of a metal such as aluminum (Al), is formed on the back surface 31a of the substrate 31. This conductive layer 36 serves as a back, electrode. The layer 36 is in Ohmic contact with the substrate 31.
When a positive electric potential with respect to the conical cathodes 35 is applied to the gate electrode 33 in a vacuum atmosphere, electrons 37 are emitted or extracted from the vicinity of the tips of the cathodes 35 due to the "field emission" phenomenon. The potential is applied to the cathodes 35 through the back electrode 36 and the substrate 31. The emitted electrons 37 movre upward along the paths 37a in the space near the gate electrode 33, traveling toward an anode (not shown) along an arrow 40.
The condition for the field emission phenomenon of the electrons 37 is determined according to the shape of the cathodes 35 and the distance between the gate electrode 33 and the corresponding cathodes 35.
With the conventional field emission device shown in FIG. 1, there is a problem that the overall emission direction 40 of the electrons 37 is largely inclined toward the left-hand side in FIG. 1 to a normal of the surface 36a of the back electrode 36, resulting in the emission direction 40 not perpendicular to the surface 36a, This problem is caused by the following fact:
Specifically, the upper conductive layer 42 is partially formed on the insulating layer 32 to be asymmnetric with the cathodes 35. Therefore, the electric field 39 in a spatial region located just over the conductive layer 42 (which is mainly positioned on the left-hand side in FIG. 1) is strongly affected by the electric potential of the conductive layer 42, not the electric potential of the substrate 31, i.e., the cathodes 35. On the other hand, the electric field in the remaining region located outside the conductive layer 42 is affected by the electric potential of the substrate 31 through the insulating layer 32,
To correct the above inclination of the overall emission direction 40 of the electrons 37, there has been known a method that an additional electrode with the same geometric shape as that of the conductive layer 42 is provided to be apart from and opposite to the layer 42. However, this method will cause another problem of an increase in parasitic capacitance.